Method of making inorganic, metal oxide spheres using microstructured molds

ABSTRACT

A process for making inorganic, metal oxide spheres that includes exposing solidified, molded microparticles that include a glass precursor composition to a temperature sufficient to transform the molded microparticles into molten glass and cooling the molten glass to form inorganic, metal oxide spheres.

This is a continuation of U.S. application Ser. No. 14/228790, filedMar. 28, 2014 (allowed), which is a continuation of U.S. patentapplication Ser. No. 11/465848, filed Aug. 21, 2006 (U.S. Pat. No.8,701,441), each of which is incorporated herein by reference in itsentirety.

BACKGROUND

The invention relates to forming inorganic, metal oxide spheres frommolded microparticles.

A variety of methods are currently used to produce glass beads, whichare also referred to as glass microspheres. These processes oftenrequire repeated steps of pulverizing and classifying particulatematerial in an effort to obtain glass beads that exhibit a relativelynarrow size distribution. Some glass bead manufacturing processesinclude generating a particulate feed material, followed by conversionof the particulate feed material to a glass by melting. The particulatefeed materials can be formed by pulverizing glass particles (orcomponents that form glass when heated at a sufficiently hightemperature) and intimately blending, e.g., by milling, the pulverizedparticles with a volatile liquid such as water. A binder such as dextrinor starch is sometimes added to bind together the milled raw materialparticles. The slurry of milled material is then dried, e.g., in bulk orby spraying the composition into a dry atmosphere maintained at anelevated temperature to yield dried feed material. In the case ofspray-drying, the dried agglomerates can then be converted directly intoglass. Feed material dried in bulk often takes the form of a cake. Adried cake can be converted to useful agglomerates by pulverizing.Optionally, the cake can be sintered before pulverizing to pre-reactsome components of the cake. In the case of bulk drying, the pulverizedagglomerates must be classified to achieve a sufficiently narrow sizerange of finished beads. Classification of the agglomerates isundesirable, due to added cost and energy usage.

Once the classified agglomerates have been generated, glass microspherescan then be formed using a variety of melting methods. In one meltingmethod, the agglomerates of raw material are passed through a flamehaving a temperature sufficient to melt the particles and through adistance sufficient to spheroidize the melted particles. For many rawmaterials exposure to a flame having a temperature of from about 1500°C. to about 2000° C. is sufficient. The melted particles are thenquenched, e.g., in air or water, to form solid beads. The quenchedparticles optionally can be crushed to form particles of a smallerdesired size for the final beads and then further processed. In othermethods, the raw material is melted and the melted material is pouredcontinuously into a jet of high velocity air. Molten droplets form asthe jet impinges on the liquid stream. The velocity of the air and theviscosity of the melt are adjusted to control the size of the droplets.The molten droplets are then rapidly quenched, e.g., in air or water, toform solid beads. Beads formed by such melting methods are normallycomposed of a vitreous material that is essentially completely amorphous(i.e., noncrystalline). The beads are often referred to as “vitreous,”“amorphous,” or simply “glass” beads or microspheres. Beads formed byliquid glass atomization often exhibit a wide size distribution,requiring classification (e.g., screening) of the product, which leadsto excess cost and energy use.

These processes often require many steps of pulverizing, classifying, orsintering to achieve particles having a desired size and sizedistribution.

SUMMARY

In one aspect, the invention features a process for making inorganic,metal oxide spheres, the process including exposing solidified, moldedmicroparticles that include a glass precursor composition to atemperature sufficient to transform the molded microparticles intomolten glass, and cooling the molten glass to form inorganic, metaloxide spheres.

In one embodiment, the process is for making glass microspheres and theprocess includes exposing solidified, molded microparticles that includea glass precursor composition to a temperature sufficient to transformthe molded microparticles into molten glass droplets, maintaining themolten glass droplets at the transforming temperature for a period oftime sufficient such that the molten glass droplets form into spheres,and cooling the molten glass droplets to form glass microspheres.

The invention features a process that facilitates production ofinorganic, metal oxide spheres (e.g., glass, glass-ceramic, glass-bondedceramic and crystalline ceramic beads) having a narrow sizedistribution. The process also enables the ability to form inorganic,metal oxide spheres having a predetermined particle size and to tune theparticle size of spheres as desired.

The relatively narrower size distribution of the molded microparticlesallows the flame used to transform the molded microparticles into moltendroplets to be adjusted to optimize performance based on the target sizeof the sphere formed there from. In the case of glass beads, the narrowsize distribution of the molded microparticles allows optimization ofthe requisite vitrification energy, resulting in a more consistent indexof refraction of the glass beads produced thereby. In some cases, theprocess can reduce or eliminate the need for the subsequent heattreatment step that is sometimes done to improve the properties ofmicrospheres.

The invention provides a process that enables a quick change over fromone sphere size, sphere chemistry or both to another relative toexisting glass bead manufacturing processes, which in some embodimentscan increase the speed of the process, decrease the waste associatedwith the change-over, and improve the overall utilization rate andpercentage yield of the system.

Other features and advantages will be apparent from the followingdescription of the preferred embodiments, the drawings, and the claims.

GLOSSARY

In reference to the invention, these terms have the meanings set forthbelow:

The term “inorganic metal oxide sphere” means glass, glass-ceramic,glass-bonded ceramic, crystalline ceramic spheres or a combinationthereof.

The term “glass” means an inorganic, metal oxide product of fusion thathas cooled to a rigid condition without crystallizing such that it isessentially amorphous, i.e., at least 95% by volume, as determined usingx-ray diffraction.

The term “glass-ceramic” means an inorganic, metal oxide formedinitially as a glass that is subsequently devitrified such that itexhibits an at least partially crystalline phase and optionally someresidual glass phase.

The term “devitrify” means to convert, at least partially from a glassystate to a crystalline state.

The term “glass-bonded ceramic” means an inorganic, metal oxide thatincludes a glassy phase and a crystalline phase.

The term “crystalline ceramic” means an inorganic, metal oxide that isessentially crystalline having less than 1% by volume glassy phase.

The term “glass precursor” means a material that is capable of formingat least one of glass, glass-ceramic, glass-bonded ceramic, andcrystalline ceramic when heated to a sufficient temperature and thencooled.

The term “handleable molded microparticle” means a molded microparticlethat has been sufficiently solidified such that it maintains its moldedshape when demolded.

The term “sphere” means a particle that is substantially, althoughperhaps not exactly, spherical and further refers to both beads andbubbles.

The term “bead” refers to a solid particle that is substantially,although perhaps not exactly, spherical.

The term “bubble” refers to a hollow particle that is substantially,although perhaps not exactly, spherical.

The term “fused” refers to preparation by a melt process.

The term “microsphere” refers to spheres having a diameter less thanabout 1 millimeter.

The term “microbead” refers to beads having a diameter less than about 1millimeter.

The term “microbubble” refers to bubbles having a diameter less thanabout 1 millimeter.

The term “molded microparticle” refers to a particle that has apredetermined shape as a result of having been formed in a mold cavityand has a volume no greater than 8,000,000,000 μm³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a process for making inorganic, metaloxide spheres using micromold tooling according to one exemplaryembodiment of the invention.

FIG. 2 is a schematic view of a process for making inorganic, metaloxide spheres using micromold tooling according to another exemplaryembodiment of the invention.

FIG. 3 is a schematic view of a process for making inorganic, metaloxide spheres using micromold tooling according to a further exemplaryembodiment of the invention.

FIG. 4 is a perspective view of a portion of a production tool.

DETAILED DESCRIPTION

The process for making inorganic, metal oxide spheres includes forming aglass precursor composition, filling a number of micromold cavities withthe glass precursor composition, solidifying (e.g., drying, curing or acombination thereof) the glass precursor composition to form moldedmicroparticles, demolding the molded microparticles from the moldcavities, passing the molded microparticles through a flame to melt themolded microparticles, which then form molten droplets as they continuepassing through a distance, quenching the molten droplets to formhardened spheres, and collecting the resulting inorganic, metal oxidespheres. The residence time in the flame and the length of the path themolten material travels are sufficient such that the molten materialforms spherical particles. The process is useful for forming inorganic,metal oxide spheres including, e.g., beads, bubbles, microspheres (e.g.,microbeads and microbubbles), and combinations thereof.

The process can be used to form spheres having a variety of dimensions.The process is particularly useful for forming spheres that arespherical or substantially spherical, i.e., a majority of the beads areround as opposed to having a nonlinear circumference or being oval oregg-shaped. The spheres can have a variety of properties including,e.g., being solid, having at least one internal void, being hollow,having surface imperfections, e.g., a surface void, and combinationsthereof. For many applications solid spheres are preferred. In otherapplications, hollow spheres are useful. The spheres can have anydesirable diameter. Preferably the mean sphere diameter is from about 10μm to about 2 mm, at least about 10 μm, at least about 20 μm, at leastabout 50 μm, at least about 55 μm, no greater than about 2 mm, nogreater than about 1 mm, no greater than about 500 μm, no greater thanabout 300 μm, no greater than about 250 μm, no greater than about 100μm, no greater than about 75 μm, or even about 60 μm. The process canalso form spheres having a relatively narrow size distribution.Preferably the size distribution of the resulting spheres is such thatthe spheres have an average absolute deviation from the mean of nogreater than about 20%, or even no greater than about 10%. The spheresoptionally can be screened to achieve a desired size distribution.

The diameter of a sphere is a function of various process parametersincluding, e.g., the size of the molded microparticles, the componentspresent in the glass precursor composition, and the degree ofdensification of the glass precursor composition.

The glass precursor composition used to form the spheres includes glassprecursor particles and optionally a vehicle that includes at least oneof water, volatile organic liquid, and fugitive binder, i.e., a binderthat dissipates during the elevated temperature processing used informing the spheres. The glass precursor particles are preferablydispersed in the vehicle such that the composition forms a dispersion(e.g., a slurry). One example of a useful glass precursor composition isone that includes glass precursor particles and water and is in the formof a slurry.

The glass precursor particles are capable of forming at least one ofglass, glass-glass-ceramic, glass-bonded ceramic, and crystallineceramic upon heating to a sufficient temperature. Useful glass,glass-ceramic, glass-bonded ceramic, and crystalline ceramic formingcompounds include metal oxides. Useful metal oxides form from a varietyof metals including, e.g., aluminum, silicon, thorium, tin, titanium,yttrium, zirconium, boron, phosphorus, germanium, lead, bismuth,tantalum, niobium, antimony, arsenic, lanthanum, gadolinium, lithium,sodium, potassium, magnesium, calcium, strontium, barium, zinc, andmixtures thereof. Useful metal oxides include, e.g., Al₂O₃, SiO₂, ThO₂,SnO₂, TiO₂, Y₂O₃, ZrO₂, B₂O₃, P₂O₅, GeO₂, PbO, Bi₂O₃, Ta₂O₅, Nb₂O₅,Sb₂O₅, As₂O₃, La₂O₃, Gd₂O₃, Li₂O, Na₂O, K₂O, MgO, CaO, SrO, BaO, andZnO, and mixtures thereof. An example of a metal whose oxides can serveas useful material in admixture with the above-mentioned oxides isberyllium. Useful metal oxides that are often colorless or only weaklycolored include, e.g., BaO, BeO, Bi₂O₃, B₂O₃, CaO, PbO, Sb₂O₅, SrO,Ta₂O₅, MgO, and ZnO and mixtures thereof. The glass precursorcomposition can also include small amounts of various metals including,e.g., iron, manganese, cobalt, vanadium, copper, nickel, tungsten,molybdenum, praseodymium, neodymium, europium, dysprosium, holmium,erbium, thulium, ytterbium, samarium, and combinations thereof. Otheruseful glass bead precursors include, e.g., metal oxide compounds thatinclude more than one metal species including, e.g., BaTiO₃,wollastonite (i.e., CaSiO₃) and combinations thereof. The glassprecursor composition can also include a color agent. Useful coloragents include, e.g., CeO₂, Fe₂O₃, CoO, Cr₂O₃, NiO, CuO, MnO₂, andmixtures thereof. The glass precursor composition can also optionallyinclude rare earth elements including, e.g., europium, for fluorescence.

The glass precursor can be provided in a variety of forms including,e.g., particulate (i.e., powder). The glass precursor is preferably inthe form of particles having a cross-sectional dimension no greater thanabout 20 μm, no greater than about 10 μm, preferably no greater thanabout 5 μm, or even from about 1 μm to about 10 μm.

The glass precursor is preferably present in the glass precursorcomposition in an amount of from about 10% by weight to about 100% byweight, from about 20% by weight to about 90% by weight, or even fromabout 50% by weight to about 85% by weight. Examples of useful glassprecursor particle mixtures are described in U.S. Pat. Nos. 3,294,558,3,493,403, 4,063,916, 4,349,456, 4,385,917, 4,564,556, 4,837,069,6,245,700, 6,335,083, and 6,914,024, and U.S. Patent Publication No.2004/0259713, all of which are incorporated herein.

The vehicle is preferably present in the glass precursor composition inan amount no greater than about 90% by weight, no greater than about 70%by weight, no greater than about 60% by weight, at least about 5% byweight, from about 0% by weight to about 50% by weight, from about 5% byweight to about 50% by weight, from about 10% by weight to about 40% byweight, or even from about 20% by weight to about 30% by weight.

Aqueous-based glass precursor compositions can include other additivesincluding, e.g., hydrocolloids (e.g., xanthan, maltodextrin,galactomannan and tragacanth) polysaccharides, natural gums (e.g., gumArabic), starch derivatives, surfactants (e.g., cationic, anionic,nonionic, and zwitterionic) including, e.g., sodium lauryl sulfatepolysorbate, and sodium 2-ethylhexyl sulfate, and combinations thereof.

Examples of useful volatile organic liquids include methanol, ethanol,isopropyl alcohol, butyl alcohol, heptane, and toluene.

Useful fugitive binders include water soluble and water dispersiblebinders including, e.g., dextrin, starch, cellulose,hydroxyethylcellulose, hydroxypropylcellulose, carboxyethylcellulose,carboxymethylcellulose, carrageenan, scleroglycan, xanthan gum, guargum, hydroxypropylguar gum and combinations thereof.

Other suitable binders include, e.g., waxes, thermoplastic polymers,radiation curable resins, i.e., resins capable of being cured byradiation energy or thermal energy, and combinations thereof. Examplesof suitable waxes include natural waxes (e.g., beeswax and vegetablewaxes (e.g., carnauba and candelilla)), synthetic waxes, mineral waxes,e.g., petroleum waxes including paraffin wax, microcrystalline wax,Fischer-Tropsch waxes, and mixtures thereof.

Useful thermoplastic polymers include, e.g., polyalkylenes, e.g.,polyolefins (polyethylene, polypropylene, and polybutylene), polyamides,polyimides, poly(phenylenediamine terephthalamide), polyesters,polyacrylates, polymethacrylates, polyvinyl alcohol, polyvinyl acetate,ethylene vinyl acetate, vinyl chloride homopolymers and copolymers, andcombinations thereof.

Curable binders are those binders that undergo crosslinking when exposedto radiation energy, thermal energy and combinations thereof. Usefulsources of radiation energy include, e.g., electron beam energy,ultraviolet light, visible light, and laser light. If ultraviolet orvisible light is utilized, a photoinitiator is preferably included inthe composition. A photoinitiator is optional when a source of electronbeam energy is utilized.

Examples of useful radiation curable binders include acrylatedurethanes, acrylated epoxies, ethylenically unsaturated compounds,aminoplast derivatives having pendant unsaturated carbonyl groups,isocyanurate derivatives having at least one pendant acrylate group,isocyanate derivatives having at least one pendant acrylate group, vinylethers, epoxy resins, and combinations thereof. The term “acrylate”includes both acrylates and methacrylates.

Examples of useful acrylated urethanes include diacrylate esters ofhydroxy terminated isocyanate extended polyesters and polyethers.

Useful acrylated epoxies include, e.g., diacrylate esters of epoxyresins including, e.g., the diacrylate esters of bisphenol A epoxyresin.

Useful ethylenically unsaturated compounds include, e.g., monomeric andpolymeric compounds that contain atoms of carbon, hydrogen and oxygen,and optionally nitrogen, halogen, and combinations thereof. At least oneof oxygen and nitrogen atoms are generally present in ether, ester,urethane, amide, and urea groups. Some useful ethylenically unsaturatedcompounds have a molecular weight of less than about 4,000 and are theester reaction product of at least one of aliphatic monohydroxy groupsand aliphatic polyhydroxy groups, and an unsaturated carboxylic acid(e.g., acrylic acid, methacrylic acid, itaconic acid, crotonic acid,isocrotonic acid, maleic acid and combinations thereof). Usefulacrylates include, e.g., methyl methacrylate, ethyl methacrylate,ethylene glycol diacrylate, ethylene glycol methacrylate, hexanedioldiacrylate, triethylene glycol diacrylate, trimethylolpropanetriacrylate, glycerol triacrylate, pentaerythritol triacrylate,pentaerythritol methacrylate, pentaerythritol tetraacrylate andcombinations thereof.

Other useful ethylenically unsaturated compounds include, e.g.,monoallyl, polyallyl, and polymethylallyl esters and amides ofcarboxylic acids including, e.g., diallyl phthalate, diallyl adipate,and N,N-diallyladipamide. Still other useful ethylenically unsaturatedcompounds include styrene, divinyl benzene, and vinyl toluene. Othernitrogen-containing, ethylenically unsaturated compounds includetris(2-acryloyl-oxyethyl)isocyanurate,1,3,5-tri(2-methacryloxyethyl)-s-triazine, acrylamide, methylacrylamide,N-methylacrylamide, N,N-dimethylacrylamide, N-vinylpyrrolidone, andN-vinylpiperidone.

Suitable aminoplast include monomeric and oligomeric aminoplast. Usefulaminoplast resins have at least one pendant α,β-unsaturated carbonylgroup per molecule. These α,β-unsaturated carbonyl groups can beacrylate, methacrylate, or acrylamide groups. Examples of such resinsinclude N-hydroxymethyl-acrylamide, N,N′-oxydimethylenebisacrylamide,ortho and para acrylamidomethylated phenol, acrylamidomethylatedphenolic novolac, and combinations thereof. These materials are furtherdescribed in U.S. Pat. Nos. 4,903,440 and 5,236,472, both of which areincorporated herein.

Examples of isocyanurate derivatives having at least one pendantacrylate group and isocyanate derivatives having at least one pendantacrylate group are described in U.S. Pat. No. 4,652,274 and incorporatedherein.

Examples of suitable vinyl ethers include vinyl ether functionalizedurethane oligomers.

Epoxies have an oxirane ring and are polymerized by the ring opening.Epoxy resins include monomeric epoxy resins and polymeric epoxy resins.These resins can vary greatly in the nature of their backbones andsubstituent groups. The backbone, for example, may be of any typenormally associated with epoxy resins and substituent groups thereon canbe any group free of an active hydrogen atom that is reactive with anoxirane ring at room temperature. Representative examples of substituentgroups for epoxy resins include halogens, ester groups, ether groups,sulfonate groups, siloxane groups, nitro groups, and phosphate groups.Examples of epoxy resins include2,2-bis[4-(2,3-epoxypropoxy)phenyl]propane (diglycidyl ether ofbisphenol A). Other suitable epoxy resins include glycidyl ethers ofphenol formaldehyde novolac. The epoxy resins can polymerize via acationic mechanism with the addition of an appropriatephotoinitiator(s). These resins are further described in U.S. Pat. Nos.4,318,766 and 4,751,138, both of which are incorporated herein.

Examples of useful photoinitiators that generate a free radical sourcewhen exposed to ultraviolet light include, e.g., organic peroxides, azocompounds, quinones, benzophenones, nitroso compounds, acyl halides,hydrazones, mercapto compounds, pyrylium compounds, triacrylimidazoles,bisimidazoles, chloroalkytriazines, benzoin ethers, benzil ketals,thioxanthones, and acetophenone derivatives, and mixtures thereof.Examples of photoinitiators that generate a free radical source whenexposed to visible radiation are described in U.S. Pat. No. 4,735,63 andincorporated herein.

Cationic photoinitiators generate an acid source to initiate thepolymerization of an epoxy resin or a urethane. Cationic photoinitiatorscan include a salt having an onium cation and a halogen-containingcomplex anion of a metal or metalloid. Other cationic photoinitiatorsinclude a salt having an organometallic complex cation and ahalogen-containing complex anion of a metal or metalloid. Usefulphotoinitiators are described in U.S. Pat. Nos. 4,751,138 and 4,985,340and European Patent Application Nos. 306,161 and 306,162, all of whichare incorporated herein. Still other cationic photoinitiators include anionic salt of an organometallic complex in which the metal is selectedfrom the elements of Periodic Groups IVB, VB, VIB, VIIB, and VIIIB.

The glass precursor composition used to form inorganic metal oxidebubbles can optionally include a blowing agent. Useful blowing agentsinclude, e.g., sulfur and compounds of oxygen and sulfur. Particularlyuseful microbubble precursor compositions are disclosed, e.g., in U.S.Pat. Nos. 4,391,646, 4,767,726 and 5,691,059, and incorporated herein.

The glass precursor composition can be prepared by combining the variouscomponents of the composition using any suitable technique including,e.g., mixing (e.g., high shear mixing), air stirring, tumbling andcombinations thereof. A vacuum can be used during mixing to minimizeentrapment of air in the glass bead precursor composition.

The micromold cavities are configured to have a volume proportional tothe desired size of the sphere formed from the molded microparticles. Avariety of factors influence the selection of an appropriate volume forthe micromold cavity including, e.g., the desired size of the resultingsphere, the dimensions of the micromold cavities, the percent solids inthe glass precursor composition, and the expected percent densification(i.e., shrinkage) resulting from the melting and/or fusing process. Toobtain a spherical bead having a diameter of about D, for example, auseful cavity volume can be determined according to the followingequation:V=4/3(π)(D/2)³÷(% solids)÷(% densification),where D is the desired diameter of the bead, % solids refers to the %solids present in the glass precursor composition, and % densificationis the amount of volume shrinkage expected from the glass precursorcomposition. Useful cavity volumes include, e.g., at least about 50,000μm³, at least about 100,000 μm³, at least about 200,000 μm³, no greaterthan about 500,000 μm³, no greater than about 300,000 μm³, no greaterthan about 1,000,000 μm³, no greater than about 8,000,000,000 μm³, oreven from about 10,000 μm³ to about 500,000 μm³.

The micromold cavity can exhibit any shape including, e.g., polyhedron(e.g., cube, prism, pyramid, tetrahedron, pentahedron, hexahedron,octahedron, decahedron, parallelepiped (e.g., rhombohedron), anddiamond), hemisphere, cylinder, arcuate, arcuate terminated cylinder,cone, frusto-conical cone, a thin body having opposed polygonal facesincluding, e.g., triangle, square, rectangle, rhomboid, pentagon,hexagon, heptagon, and octagon faces, gumdrop, bell, and combinationsthereof.

The micromold cavity can exist in any suitable substrate. Preferably anumber of micromold cavities are present in a substrate. The substratein which the micromold cavities are present is referred to herein as a“production tool.” The production tool can be a three-dimensional bodyhaving at least one continuous surface. The continuous surface containsat least one opening, preferably a number of openings, formed in thecontinuous surface. Each opening provides access to a cavity formed inthe three-dimensional body. The production tool can be of a variety ofconstructions including, e.g., a web, e.g., an endless belt, a sheet, aroll (e.g., a coating roll), a sleeve mounted on a coating roll, andcombinations thereof. The production tool can be constructed to allowfor continuous operation including, e.g., endless belts and cylindricalcoating rolls that rotate about an axis. Examples of useful endless beltproduction tools are described in U.S. Pat. No. 5,549,962 andincorporated herein. Apparatus utilizing a two-ended web can also beadapted to provide continuous operations.

A single production tool can contain a number of cavities having thesame size and shape, having different shapes, having different sizes,and combinations thereof. In the case where the substrate is in the formof a web or a belt, the cavity can extend completely through theproduction tool. The cavities can abutt or have land areas between them.Increasing the amount of land area between cavities can assist inpreventing bridging of the glass bead precursor composition from onemold cavity to another. Sloped sides on cavities can provide a varietyof advantages including, e.g., easier filling of the production tool,easier removal of the solidified, molded microparticle from theproduction tool, and combinations thereof.

The production tool can be constructed from a variety of materialsincluding, e.g., metals (e.g., nickel), polymers (e.g., polyolefins,e.g., polypropylene, polyamide, polyimide and combinations thereof),ceramic materials, and combinations thereof. A production tool made ofmetal can be fabricated by diamond turning, engraving, photolithography,hobbing, etching, knurling, assembling a plurality of metal partsmachined in the desired configuration, die punching, other mechanicalmeans, electroforming, and combinations thereof. Useful techniques forfabricating production tools are described in the Encyclopedia ofPolymer Science and Technology, Vol. 8, John Wiley & Sons, Inc. (1968),p. 651-665, and U.S. Pat. No. 3,689,346, both of which are incorporatedherein.

The metal production tool may include a release agent (e.g., a releasecoating) on the surface of the mold, e.g., the mold cavities, to enableeasier removal of the molded microparticles from the cavities and tominimize wear of the production tool. Examples of suitable releasecoatings include hard coatings such as metal carbides, metal nitrides,metal borides, diamond, diamond-like carbon, and combinations thereof.

A metal production tool can also be treated, e.g., by heating, cooling,and combinations thereof. The temperature of the tool may allow easierprocessing, more rapid curing, and easier release of the shapedmicroparticles from the tool.

Polymeric production tools can be manufactured using a variety oftechniques. Some polymeric production tools are replicated from anoriginal master tool. Polymeric tools can be made to allow radiation topass from the radiation source through the production tool and into theglass bead precursor composition, which is particularly useful for glassbead precursor compositions that include a radiation curable component.Polymeric production tools can also be prepared by coating a moltenthermoplastic resin, such as polypropylene, onto a master tool. A metalmaster tool can be made by the same methods used to make metalproduction tools. The molten resin is then quenched to give athermoplastic replica of the master tool. This polymeric replica canthen be utilized as the production tool. If the production tool is madefrom a thermoplastic material, the conditions of the method in which thetool is used should be set such that processing conditions do notadversely affect the production tool.

The polymeric production tool may also optionally include a releaseagent to improve the releasability of the molded microparticle from theproduction tool. The release agent can be in a variety of formsincluding, e.g., a release coating on the surface of the tool (e.g., thesurface of the cavities), a release agent incorporated into thecomposition of the tool, and combinations thereof. Useful release agentcoating compositions include, e.g., silicone-based compositions,fluorochemical-based compositions, and combinations thereof. A releaseagent optionally can be present in the polymer from which the productiontool is formed. Useful release agents include silicone-based materialsand fluorochemical-based materials. Alternatively or in addition theproduction can include a thermoplastic polymer that exhibits releaseproperties, an example of which is described in WO 92/15626, andincorporated herein.

Other methods of preparing production tools are described in U.S. patentapplication Ser. No. 08/004,929, filed Jan. 14, 1993, now abandoned, andincorporated herein.

The glass precursor composition can be placed in the micromold cavityusing any suitable technique including, e.g., gravity feeding, pumping,coating (e.g., die coating, knife coating, spray coating), vacuum dropdie coating, and combinations thereof. Alternatively or in addition, theglass bead precursor can be introduced to the cavities of the productiontool by transfer via a carrier web.

Although the glass precursor composition is only required to fill aportion of the cavity, the glass precursor composition preferablycompletely fills the cavity in the surface of the production tool sothat the resulting molded microparticle will contain few voids orimperfections or be free of the same. Imperfections can alter the sizeof the molded microparticle, which can impact the size distribution ofthe spheres formed therefrom.

It is sometimes preferred to alter the viscosity of the glass precursorcomposition prior to introducing the composition into the cavity ofmicromold. Various methods can be used to lower the viscosity of theglass precursor composition prior to or during the filling processincluding, e.g., heating the composition prior to introducing thecomposition into the production tool (e.g., to a temperature in therange of from about 40° C. to 90° C.) so that it can flow more readilyinto the cavities of the production tool, subjecting the glass beadprecursor composition to ultrasonic energy (e.g., vibration) during themixing step or immediately prior to the coating step, applying a vacuumor pressure, rolling bank, adding liquid to the composition, andcombinations thereof.

The filled micromold can optionally undergo a scraping process to removeexcess glass precursor composition from the micromold cavity, from anyland area that exists between the micromold cavities and combinationsthereof. The scraping process can occur at any suitable time including,e.g., substantially simultaneously with the filling of a number ofcavities, subsequent to filling a number of cavities, and combinationsthereof.

The glass precursor composition is then at least partially solidifiedwhile in the micromold cavities. Partial solidification can include,e.g., drying, curing (e.g., crosslinking) and combinations thereof. Anysuitable method can be used to solidify the glass precursor compositionincluding, e.g., ambient drying, drying in an oven, exposure to thermalenergy, exposure to radiation energy, and combinations thereof.

Useful drying conditions for an aqueous-based glass precursorcomposition include, e.g., heating the precursor composition to atemperature sufficient to solidify the composition to a point such thatit is at least handleable, exposing the composition to radiationincluding, e.g., infrared radiation, ultraviolet (uv) radiation,electron beam radiation, and microwave radiation, and combinationsthereof.

For glass precursor compositions that include a curable binder, theglass precursor composition can be at least partially cured (e.g.,crosslinked) while it is present in the cavities of the production tool,and then, optionally, post-cured after the molded glass precursor isremoved from the cavities of the production tool. The degree of cure issufficient such that the resulting solidified, handleable glass beadprecursor will retain its shape upon removal from the production tool.

Examples of sources of radiation energy for use in the curing zoneinclude electron beam, ultraviolet light, visible light, microwaveenergy, infrared radiation, and laser light and combinations thereof.The amount of energy and duration of exposure can be selected based on avariety of factors including, e.g., the chemistry of the glass precursorcomposition, the speed of the carrier on which the composition is beingtransported, the distance of the radiation source from the carrier, theposition of the radiation source relative to the carrier (e.g., theradiation source may be positioned so as to transmit through a carrier),ambient conditions, and combinations thereof.

After being at least partially solidified, the resulting solidified,handleable molded glass precursor composition can be demolded from theproduction tool to provide a molded microparticle. A given moldedmicroparticle will have a shape that is essentially the shape of themold cavity of the production tool in which the molded microparticle hasbeen at least partially solidified. An advantage of this mode is thatthe molded microparticles are already of the proper size distribution,volume and shape for subsequent use. The predetermined size and shape ofthe molded microparticles also aids in screening and flow of the moldedmicroparticles. Useful molded microparticle shapes include, e.g.,polyhedron (e.g., cube, prism, pyramid, tetrahedron, pentahedron,hexahedron, octahedron, decahedron, parallelepiped (e.g., rhombohedron),and diamond), cylinder, arcuate, arcuate terminated cylinder,hemisphere, gumdrop, bell, conical, frusto-conical, thin body havingopposed polygonal faces including, e.g., triangle, square, rectangle,rhomboid, pentagon, hexagon, heptagon, and octagon faces, andcombinations thereof.

Any suitable method can be used to remove the molded microparticle fromthe mold cavity. For production tools that are made of a polymericmaterial, one useful demolding method includes exposing the filledproduction tool to sonic energy. Other useful demoldng methods include,e.g., static charge, vacuum, air knife, other mechanical means, andcombinations thereof.

For production tools made of metal, the molded microparticle can beremoved from the cavities by a water jet, air jet, and combinationsthereof. If the production tool has cavities that extend completelythrough the production tool, e.g., if the production tool is a belthaving perforations extending completely therethrough, the moldedmicroparticle can be removed by ultrasonic energy, mechanical force,water jet, air jet, combinations thereof, and other means, regardless ofthe material of construction of the production tool.

Alternatively, the molded microparticle is released from the productiontool as a sheet that includes precisely shaped molded microparticlesinterconnected by a thin layer of binder material. The binder is thenbroken or crushed along the thin interconnecting portions to form themolded microparticles.

The molded microparticles can then be transferred directly to a sourceof thermal energy such as a flame. Alternatively or in addition, themolded microparticles are transferred from the production tool to acollector, from a production tool to a smooth roll, and combinationsthereof, and then transferred to the thermal energy source. With respectto the smooth roll process, the molded microparticles exhibit greateradhesion to the smooth roll than to the production tool. The transferredmolded microparticles are then removed from the smooth roll by varioustechniques including, e.g., skiving, vacuum, water jet, air jet, othermechanical means, and combinations thereof. In one particularembodiment, the molded microparticles are transferred from theproduction tool to a major surface of a carrier web. The moldedmicroparticles exhibit greater adhesion to the major surface of thecarrier web than to the production tool. The major surface of thecarrier web to which the molded microparticles are transferred can beara layer of material that is soluble in water or an organic solvent. Themolded microparticles are then removed from the carrier web bydissolving the soluble layer, optionally in combination with amechanical means including, e.g., skiving, vacuum, ultrasound andcombinations thereof. In another methods, ultrasonic energy is applieddirectly over a major surface of the web or off to a side of a majorsurface of the web to release the molded microparticles therefrom.

In another embodiment of a method that employs a carrier web, the majorsurface of the carrier web includes a primer. The molded microparticleswill preferentially adhere to the primed carrier web. The moldedmicroparticles can then be removed from the primed carrier web by anysuitable means including, e.g., skiving, vacuum, ultrasound, andcombinations thereof. Examples of suitable primers include ethyleneacrylic acid copolymer, polyvinylidene chloride, crosslinked hexanedioldiacrylate, aziridine materials, and combinations thereof.

The volume of a molded microparticle preferably is at least about 50,000μm³, at least about 100,000 μm³, at least about 200,000 μm³, no greaterthan about 500,000 μm³, no greater than about 300,000 μm³, no greaterthan about 1,000,000 μm³, no greater than about 8,000,000,000 μm³, oreven from about 10,000 μm³ toabout 500,000 μm³.

The molded microparticles are then passed through a flame or othersource of sufficient thermal energy (e.g., a gas-fired furnace or anelectrical furnace) to form molten glass droplets. Any suitable sphereforming process and apparatus can be used including, e.g., glass,glass-ceramic, glass-bonded ceramic, and crystalline ceramic spheresmanufacturing processes and apparatuses.

In one useful method, the molded microparticles are in the form of afree flowing powder and the passing involves allowing the free flowingpowder to be dispersed in a flame. The flame is preferably positionedhorizontally and has a temperature sufficient to transform, e.g., fuse,the glass precursors present in the molded microparticle into ahomogenous state. The flame temperature is selected to be suitable formelting and fusing the molded microparticles into glass droplets. Usefulflame temperatures are at least about 2000K, at least about 3000K, oreven from about 3000K to about 5000K. The flame can be generated by anysuitable fuel and oxidant sources including, e.g., natural gas,hydrogen, oxygen, acetylene, air, and mixtures thereof.

The duration of the molded microparticles in the flame is referred to as“residence time.” The residence time is selected to achieve sphereshaving a desired property(s). Variables that impact the residence timeinclude, e.g., flame velocity, flame size, flame shape, flametemperature, molded microparticle volume, the composition of the moldedmicroparticle, the density of the molded microparticle, and the densityof the sphere. The molten droplets can be maintained in the flame for asufficient period of time to transform the molten droplets into spheresthrough any suitable mechanism including, e.g., directing gas currentsunder the molten droplets, allowing the molten droplets to fall freelythrough the heating zone, and combinations thereof.

The fused glass droplets form spheroids, which are then quenched to formspheres. Various quenching methods are suitable including, e.g., aircooling (e.g., by free falling through a space a sufficient distance),rapid cooling and combinations thereof. A useful rapid cooling methodincludes allowing the spheroids to continue their free fall through acooling zone or into a cooling medium, e.g., water, oil or a combinationthereof. Alternately or in addition, a gas (e.g., air or argon) can besprayed into the free falling stream of fused spheroids causing thespheroids to accelerate and cool forming solid, transparent glassmicrobeads.

The spheres are then collected and, where desired, further processedincluding, e.g., screening (which is also referred to as classifying,sieving and sizing), heat treating (e.g., to allow the spheres todevelop crystallinity, to form glass-ceramic, glass-bonded ceramic, andcrystalline ceramic spheres and combinations thereof), fully ceramic,and combinations thereof. Useful heat treating methods are disclosed,e.g., in U.S. Pat. No. 6,245,700 and incorporated herein.

FIG. 1 illustrates an embodiment of the process 10 for making beads 34in which the production tooling 12 in the form of a web that includesmicromold cavities travels past a feed station 16 and to other stationswith the aid of action by a series of rollers 18 a-i. A glass precursorcomposition 14 in the form of a slurry that includes glass precursor andwater is fed from a milling station 20 to the feed station 16 and fromthe feed station 16 to the cavities in the production tooling 12. Theglass precursor composition 14 filled tooling 12 travels from the feedstation 16 to the drying station 22 where the glass precursorcomposition 14 in the cavities is solidified to form moldedmicroparticles 28. The glass precursor composition 14 filled tooling 12then travels to a release station 24 where the output from a sonic horn26 causes the molded microparticles 28 to be released from the cavitiesof the tooling 12. The molded microparticles 28 then fall a distance toa flame forming station 30 where they are melted by a flame 32 and formmolten droplets. As the molten droplets continue to fall through theflame forming station 30 they harden into beads 34 and are collected.

FIG. 2 illustrates another embodiment of a process 50 for making beads46. In this embodiment, the process proceeds as described above inreference to FIG. 1 with the exception that after the moldedmicroparticles 28 are released from the cavities of the tooling 12 atrelease station 24 the molded microparticles 28 are collected in acontainer 27 and stored for future processing. Molded microparticles 28stored in the container 27 are then fed to a flame forming station 42where the molded microparticles 28 are melted by a flame 44 and formmolten droplets. As the molten droplets continue to pass through theflame forming station 42 they harden into beads 46 and are collected.The flame forming station 42 optionally is operably coupled to thecontainer 27. The container 27 and the flame forming station 42 can beoperably coupled to each other using any suitable mechanism including,e.g., a belt feeder, to form a continuous operation.

FIG. 3 illustrates a further embodiment of process 60 for making beads78 in which the production tooling 63, which includes micromoldcavities, is in the form of an endless belt that travels past feedstation 56 and to other stations with the aid of action by two rollers68 a-b, at least one of which is power driven. A glass precursorcomposition 52 is fed from a milling station 54 to feed station 56 andfrom feed station 56 to the cavities in the tooling 63. The glassprecursor composition 52 filled tooling 63 travels from feed station 56to drying station 62 where the glass precursor composition 52 in thecavities is solidified to form molded microparticles 70. The filledtooling 63 then travels to a release station 64 where the output from asonic horn 66 causes the molded microparticles 70 to be released fromthe cavities of the tooling 63. In this embodiment, the moldedmicroparticles 70 that are released from the cavities of the tooling 63at release station 64 are collected in a container 72 and stored forfuture processing. The molded microparticles 70 are then fed to a flameforming station 74 where the molded microparticles 70 are melted by aflame 76 and form molten droplets. As the molten droplets continue theirpassage through the flame forming station 74 they harden into beads 78and are collected. The flame forming station 74 optionally is operablyconnected to the container 72 by a suitable mechanism including, e.g., abelt feeder, to enable a continuous operation. Alternatively, the moldedmicroparticles 70 released at the release station 64 can be fed directlyinto the flame forming station 74, as depicted in FIG. 1.

FIG. 4 illustrates an embodiment of a production tool 36 that is athree-dimensional body having at least one continuous surface 38 and anumber of openings formed in the continuous surface 38. Each openingprovides access to a mold cavity 40 formed in the three-dimensionalbody.

Spheres having a variety of properties can be formulated and preparedfor use in a variety of compositions, applications, and constructionsincluding, e.g., coatings (e.g., paints), exposed lens sheeting,encapsulated lens sheeting, embedded lens sheeting, protectivematerials, reflective sheeting, retroreflective sheeting,pavement-marking sheet materials (e.g., tapes), useful examples of whichare described in U.S. Pat. No. 4,248,932 and incorporated herein, andother reflective and retroreflective articles including, e.g.,assemblies. Examples of various useful sheeting constructions aredisclosed in U.S. Pat. Nos. 2,407,680, 2,354,018 and 2,326,634, all ofwhich are incorporated herein. Suitable assemblies are disclosed, e.g.,in U.S. Pat. Nos. 5,310,278, 5,286,682, 5,268,789, and 5,227,221, all ofwhich are incorporated herein.

In some embodiments, retroreflective articles can be used on clothing toincrease the visibility of the individual by retroreflecting theincident light. The retroreflective articles that are utilized onclothing include strips of tape that are adhered to clothing with heatsensitive adhesive, patches permanently affixed or sewn to the clothing,and articles of clothing that include a retroreflective article withinthe clothing. Retroreflective clothing is especially useful toconstruction workers and exercisers who utilize roadways because theseindividuals are in close proximity to moving vehicles on a regular basisand the retroreflective articles make the individuals more visible todrivers under low light conditions. Various embodiments ofretroreflective articles are further described, e.g., in U.S. Pat. Nos.4,025,159; 4,983,436; 5,064,272; 5,066,098; 5,069,964; 5,262,225;5,674,605; 5,812,317; and 6,153,128.

Sheeting useful for pavement markings often includes, e.g., a backing, alayer of binder, and a layer of beads partially embedded in the layer ofbinder material. The backing can be made from a variety of materialsincluding, e.g., polymeric films, metal foils, fiber-based sheets andcombinations thereof. Suitable polymers for forming films include, e.g.,acrylonitrile-butadiene polymers, millable polyurethanes, neoprenerubber, epoxides, and combinations thereof. The backing can also includeparticulate fillers, skid resistant particles and combinations thereof.The binder can include a variety of materials including, e.g., vinylpolymers, polyurethanes, epoxides, polyesters, colorants (e.g.,inorganic pigments) and combinations thereof. The pavement markingsheeting can also include an adhesive composition (e.g., a pressuresensitive adhesive, a contact adhesive, hot melt adhesive, heatactivated adhesives, and combinations thereof), on the pavementcontacting surface of the backing sheet. Examples of useful pavementmarking constructions and materials are described in U.S. Pat. Nos.2,354,018, 3,915,771, 4,117,192, 4,248,932 and 4,490,432, all of whichare incorporated herein.

Pavement marking sheeting can be made by a variety of known processes. Arepresentative example of such a process includes coating onto a backingsheet a mixture of resin, pigment, and solvent, dropping spheres ontothe wet surface of the backing, and curing the construction. A layer ofadhesive can then be coated onto the bottom of the backing sheet. Oneexample of a useful process for making a pavement marking sheet isdescribed in U.S. Pat. No. 4,248,932 and incorporated herein.

In some embodiments, the spheres of the present disclosure can have anindex of refraction of at least about 1.2 and no greater than about 3.0.It may be preferred that the spheres have an index of refraction of atleast about 1.6 and no greater than about 2.7. It may be more preferredthat the spheres have an index of refraction of at least about 1.7 andno greater than about 2.5.

The spheres can be incorporated into a variety of coating compositionsas described, e.g., in U.S. Pat. Nos. 3,410,185, 3,228,897 and2,963,378, all of which are incorporated herein. The spheres can also beused in drop-on applications for painted lines as in pavement markings.

Inorganic metal oxide bubbles are also useful in a variety ofapplications and compositions including, e.g., as a lightweight fillerin molded polymeric products (e.g., injection-molded andextrusion-molded parts), oil-well cements (e.g., inorganic cementitiousmaterials that harden when mixed with water) concrete, plasters, wallrepair compounds, resins, paints and ceramic articles, and as a fillingmaterial in cavity walls for thermal insulation purposes. The bubblesalso can be sintered together to form cellular articles including, e.g.,glass bricks and other structures. The bubbles can also be filled withgaseous contents under pressure.

The invention will now be described by way of the following examples.

EXAMPLES Example 1

A 52% solids slurry was prepared as follows: a powder was prepared bycombining from 35.00 g titanium dioxide, 11.00 g silicon dioxide, 59.20g barium carbonate, 10.71 g calcium carbonate, and 3.81 g sodium boratewith mixing. To 96.07 g of the powder was added 54 g deionized water toform a 64% solids slurry. The slurry was placed in a porcelain mill jarwith zirconium media and milled for four days. An additional 10 g ofdistilled water was added on the third day of milling and 25 g deionizedwater was added on the fourth day for a total of 89 g of water resultingin a slurry having 52% solids. Then 1.92 g surfactant and 8.96 g Dsodium silicate liquid binder (PQ Corporation, Berwyn, Pa.) were addedto the slurry.

The resulting composition was then spread by hand using a doctor bladeacross a tool having 2.4×10⁷ cc pyramidal micromolds to fill the molds.The filled molds were set in an oven at 204.8° F. (96° C.) and dried forthree hours. The molded microparticles were then removed from the moldby exposure to a sonic horn.

The molded microparticles were then fed through a one inch innerdiameter copper tube with a 180 μm screen at the mouth to a flame. Theparticles were allowed to flow by gravitation down the tube to the flameand into a stream of running water cascading down a metal incline andinto a catch pan. The copper tube was tapped to facilitate flow of theparticles through the tube to a flame where they formed into glassbeads.

The flame was generated by a PM2D Model B Bethlehem Bench Burner havingan inner ring with approximately 8.0 standard liters per minute (SLPM)hydrogen and 3.0 SLPM oxygen and an outer ring having 23.0 SLPM hydrogenand 9.8 SLPM oxygen. The flame was angled across and down the incline.

The resulting glass beads had an average diameter of 51.67 microns witha standard deviation of 11.65 and a refractive index ranging from 1.810to 1.95, where 90% of the beads had a refractive index in the range of1.92 to 1.95 and a majority of the beads had a refractive index of 1.94.The beads were clear. Some of the beads included voids.

What is claimed is:
 1. A process for making inorganic, metal oxidespheres, the process comprising: forming a glass precursor compositioncomprising glass precursor particles dispersed in a vehicle such thatthe composition forms a dispersion, the vehicle including at least oneof water, an organic liquid, and a binder; positioning the glassprecursor composition in a plurality of mold cavities; solidifying theglass precursor composition in the plurality of mold cavities to formsolidified, molded microparticles; demolding the solidified, moldedmicroparticles from the plurality of mold cavities; exposing thesolidified, molded microparticles, after demolding, to a temperaturesufficient to transform the molded microparticles into molten droplets;and cooling the molten droplets to form inorganic, metal oxide spheres.2. The process of claim 1, wherein the inorganic, metal oxide spherescomprise glass.
 3. The process of claim 1, wherein the exposingcomprises: passing the molded microparticles through a chamber having atemperature sufficient to melt the solidified, molded microparticles;melting the molded microparticles; and rendering the melted particlessubstantially spherical.
 4. The process of claim 1, further comprisingmaintaining the molten droplets at or above the temperature sufficientto transform the molded microparticles into molten droplets for asufficient period of time, such that the molten droplets form intospherical molten droplets.
 5. The process of claim 1, wherein theexposing comprises exposing the molded microparticles to a temperatureof at least 2000K.
 6. The process of claim 1, wherein solidifying theglass precursor composition includes exposing the composition to anenergy source to at least partially solidify the composition.
 7. Theprocess of claim 1, wherein the vehicle comprises a binder comprising atleast one of cellulose, thermoplastic polymer, and actinic radiationcurable resin.
 8. The process of claim 1, wherein each mold cavitydefines a volume no greater than about 880,000,000 cubic micrometers. 9.The process of claim 1, wherein each solidified, molded microparticlehas a volume of no greater than 8,000,000,000 μm³.
 10. The process ofclaim 1, wherein the plurality of mold cavities is present on athree-dimensional body having at least one continuous surface comprisinga plurality of openings at least some of which provide access to themold cavities, the mold cavities extending into the three-dimensionalbody.
 11. The process of claim 1, wherein the glass precursor comprisesparticles comprising oxides of at least one of silicon, aluminum,zirconium, titanium, boron, lanthanum, sodium, potassium, calcium,magnesium, and barium.
 12. The process of claim 1, wherein theinorganic, metal oxide spheres have an average cross-sectional dimensionno greater than about 500 micrometers.
 13. The process of claim 1,wherein the inorganic, metal oxide spheres have an averagecross-sectional dimension no greater than 100 micrometers.
 14. Theprocess of claim 1, wherein the molded microparticles exhibit a shapecomprising at least one of polyhedron, parallelepiped, diamond,cylinder, arcuate, arcuate terminated cylinder, sphere, hemisphere,gumdrop, bell, cone, and frusto-conical cone.
 15. The process of claim1, wherein the inorganic, metal oxide spheres have a mean spherediameter, and wherein the inorganic, metal oxide spheres exhibit anaverage absolute deviation from the mean sphere diameter of less than20% prior to a screening step.
 16. The process of claim 1, wherein theinorganic, metal oxide spheres have a mean sphere diameter, and whereinthe inorganic, metal oxide spheres exhibit an average absolute deviationfrom the mean sphere diameter of less than 10% prior to a screeningstep.
 17. The process of claim 1, wherein the inorganic, metal oxidespheres have an index of refraction of at least about 1.2.
 18. Theprocess of claim 1, wherein the inorganic, metal oxide spheres comprisemicrobubbles or microbeads.
 19. The process of claim 1, wherein theplurality of mold cavities is formed in a production tool, and whereinthe production tool is formed of a polyolefin, a polyamide, a polyimide,a metal, a ceramic, or a combination thereof.
 20. The process of claim1, wherein the vehicle is present in the glass precursor composition inan amount of at least 20% by weight.
 21. The process of claim 1, whereinplacing the glass precursor composition in the plurality of moldcavities includes at least one of partially filling one or more of theplurality of mold cavities and completely filling one or more of theplurality of mold cavities.
 22. The process of claim 1, wherein theplurality of mold cavities includes mold cavities that abut.
 23. Theprocess of claim 1, wherein the plurality of mold cavities includes moldcavities of different shapes, different sizes, or a combination thereof.24. The process of claim 1, wherein the glass precursor composition isin the form of a slurry.
 25. The process of claim 1, wherein exposingthe solidified, molded microparticles, after demolding, to a temperaturesufficient to transform the molded microparticles into molten dropletsincludes exposing the solidified, molded microparticles, afterdemolding, to a flame.
 26. The process of claim 25, further comprisingadjusting the flame based on a target size of the inorganic, metal oxidespheres.
 27. The process of claim 1, further comprising partiallyembedding the inorganic, metal oxide spheres in a layer of bindermaterial.
 28. A process for making a retroreflective article, theprocess comprising: partially embedding the inorganic, metal oxidespheres formed according to the process of claim 1 in a layer of bindermaterial.
 29. The process of claim 1, wherein the molten dropletscomprise homogeneous molten droplets.